Top contact resistance measurement in vertical fets

ABSTRACT

A test device includes a diode junction layer having a first dopant conductivity region and a second dopant conductivity region formed within the diode junction layer on opposite sides of a diode junction. A first portion of vertical transistors is formed over the first dopant conductivity region as a device under test, and a second portion of vertical transistors is formed over the second dopant conductivity region. A common source/drain region is formed over the first and second portions of vertical transistors. Current through the first portion of vertical transistors permits measurement of a resistance at a probe contact connected to the common source/drain region.

BACKGROUND Technical Field

The present invention generally relates to semiconductor testing, andmore particularly to devices and methods for contact resistancemeasurements in vertical transistor devices.

Description of the Related Art

Precisely measuring source/drain (S/D) contact resistance is a desirablefor advancing complementary metal oxide semiconductor (CMOS) technology.Many traditional methods for measuring S/D contact resistance in planarCMOS devices cannot be employed for vertical field effect transistors(VFETs) due to drastic structural differences.

SUMMARY

In accordance with an embodiment of the present principles, a testdevice includes a diode junction layer having a first dopantconductivity region and a second dopant conductivity region formedwithin the diode junction layer on opposite sides of a diode junction. Afirst portion of vertical transistors is formed over the first dopantconductivity region as a device under test, and a second portion ofvertical transistors is formed over the second dopant conductivityregion. A common source/drain region is formed over the first and secondportions of vertical transistors. Current through the first portion ofvertical transistors permits measurement of a resistance at a probecontact connected to the common source/drain region.

Another test device includes a diode junction layer having a firstdopant conductivity region and a second dopant conductivity regionformed within the diode junction layer on opposite sides of a diodejunction, the first dopant conductivity region including a verticaltransistor drain for a device under test. A first portion of verticaltransistors is formed over the first dopant conductivity region as thedevice under test, and a second portion of vertical transistors isformed over the second dopant conductivity region. The verticaltransistors are formed vertically along semiconductor fins such that adevice channel for the vertical transistors is disposed in a normaldirection relative to the diode junction layer. A common source/drainregion is formed over the first and second portions of verticaltransistors. The common source/drain region includes a verticaltransistor source for the device under test, wherein current through thefirst portion of vertical transistors permits measurement of aresistance at a probe contact connected to the common source/drainregion.

A method for forming a test device includes forming a diode junctionlayer on a substrate, the diode junction layer having a first dopantconductivity region and a second dopant conductivity region formedwithin the diode junction layer on opposite sides of a diode junction;forming a first portion of vertical transistors over the first dopantconductivity region as a device under test and a second portion ofvertical transistors over the second dopant conductivity region; andforming a common source/drain region over the first and second portionsof vertical transistors such that current through the first portion ofvertical transistors permits measurement of a resistance at a probecontact connected to the common source/drain region.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view showing a substrate having a bottomsource/drain layer and a channel layer formed thereon in accordance withthe present principles;

FIG. 2 is a cross-sectional view showing the channel layer of FIG. 1patterned to form fins in accordance with the present principles;

FIG. 3 is a cross-sectional view showing a structure of FIG. 2 where aportion of the source/drain layer is implanted with dopants of anopposite conductivity in accordance with the present principles;

FIG. 4 is a cross-sectional view showing the structure of FIG. 3 whereshallow trench isolation regions, vertical transistors, a gate structureand a common source/drain region are formed in accordance with thepresent principles;

FIG. 5 is a cross-sectional view showing the structure of FIG. 4 wherecontacts are formed through an interlevel dielectric layer in accordancewith the present principles;

FIG. 6 is a cross-sectional view showing a test device where sensepotential is measured by holding a source voltage to zero with fins in afirst orientation in accordance with the present principles;

FIG. 7 is a cross-sectional view showing a test device where sensepotential is measured by holding a source voltage to zero with fins in asecond orientation in accordance with the present principles; and

FIG. 8 is a flow diagram showing methods for forming a test device inaccordance with the present principles.

DETAILED DESCRIPTION

In accordance with the present principles, structures and methods forfabricating the structures are provided for building a testing device.In useful embodiments, the testing device includes fins with verticalfield effect transistors (VFETs) formed along with a structure tomeasure contact resistance. The testing device may be formed on eachchip or may be formed on one or more chips of a wafer to provide datafor all chips on the wafer.

The testing setup provides a reverse biased diode junction belowvertical transistors of a device under test (DUT) and a test structure.One side of the junction is below the vertical transistors of the DUT,and the other side of the junction is below the vertical transistors ofthe test structure. A doped region having a same dopant conductivity asthe junction below the DUT is formed over the vertical transistors forthe DUT and the test structure. Equal potential is provided at a probepoint on the DUT and on a source region of the test structure to ensuregreater test measurement accuracy.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, may be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein may be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers may also be present.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, the formation of a testdevice 10 may begin with a substrate 12. The substrate 12 and itsprocessing, as described herein, will illustratively be for N-type fieldeffect transistor (NFET) devices. The NFET device will employ N-typedopants in P-wells. It should be understood that the present principlesmay also apply to P-type field effect transistor (PFET) devices byemploying P-type dopants and N-wells.

The substrate 12 includes a semiconductor material, such as, e.g., Si,SiGe, SiC, Ge, III-V materials, etc. The substrate 12 includes a P-typesubstrate, preferably Si, in this embodiment. A source/drain (S/D) layer14 is grown on the substrate 12. The S/D layer 14 is a highly dopedN-type layer (N+). The S/D layer 14 may be epitaxially grown on thesubstrate 12 and may be doped in-situ.

S/D epitaxy of layer 14 can be done by ultrahigh vacuum chemical vapordeposition (UHVCVD), rapid thermal chemical vapor deposition (RTCVD),metalorganic chemical vapor deposition (MOCVD), low-pressure chemicalvapor deposition (LPCVD), limited reaction processing CVD (LRPCVD),molecular beam epitaxy (MBE), etc. Epitaxial materials may be grown fromgaseous or liquid precursors. Epitaxial materials may be grown usingvapor-phase epitaxy (VPE), molecular-beam epitaxy (MBE), liquid-phaseepitaxy (LPE), or other suitable process. Epitaxial silicon, silicongermanium (SiGe), and/or carbon doped silicon (Si:C) silicon can bedoped during deposition (in-situ doped) by adding dopants, N-typedopants (e.g., phosphorus or arsenic) or P-type dopants (e.g., boron orgallium), depending on the type of transistor. The dopant concentrationin the source/drain layer 14 can range from about 1×10¹⁹ cm⁻³ to about2×10²¹ cm⁻³, or preferably between 2×10²⁰ cm ⁻³ and 1×10²¹ cm⁻³.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown,” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline over layer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases are controlled, and the systemparameters are set so that the depositing atoms arrive at the depositionsurface of the semiconductor substrate with sufficient energy to moveabout on the surface such that the depositing atoms orient themselves tothe crystal arrangement of the atoms of the deposition surface.Therefore, an epitaxially grown semiconductor material has substantiallythe same crystalline characteristics as the deposition surface on whichthe epitaxially grown material is formed. For example, an epitaxiallygrown semiconductor material deposited on a {100} orientated crystallinesurface will take on a {100} orientation. In some embodiments, epitaxialgrowth and/or deposition processes are selective to forming onsemiconductor surface, and generally do not deposit material on exposedsurfaces, such as silicon dioxide or silicon nitride surfaces.

A channel layer 16 is formed on the S/D layer 14. The channel layer 16may be formed by an epitaxial deposition process as described above. Thechannel layer 16 may be doped in-situ and have a lower dopantconcentration than the S/D layer 14. The material of the channel layer16 may be the same as the S/D layer 14. The channel layer 16 will beformed into fins.

Referring to FIG. 2, fins 18 may be formed by patterning a mask andetching. The etch mask may include resist or other materials, and thepatterning may include lithography, spacer image transfer (SIT) or othertechniques. The fins 18 are etched in accordance with the etch mask toform appropriate dimensions (e.g., height, width). The etching includesa reactive ion etch (RIE) or other directional etch process. The etchstops on the S/D layer 14. Note six fins 18 are depicted in FIG. 2;however, it should be understood that other numbers of fins may beemployed.

Referring to FIG. 3, a block mask (not shown) is formed over some (onehalf) of the fins 18, and the remaining fins 18 and unblocked portion ofthe S/D layer 14 are exposed to an ion implantation process. The ionimplantation process includes P-type dopant implantation to render aregion 20 a P-type region. The P-type dopants may include boron,gallium, etc. The ion implantation forms an N-P junction 22. Note thatnot every device on a wafer needs to have a test device 10 formedthereon nor does each device on a chip need a test device 10.

Referring to FIG. 4, shallow trenches are formed through the S/D layer14 (and region 20) and into the substrate 12. The shallow trenches maybe patterned using lithography or other patterning techniques and etchedwith RIE. The shallow trenches are filled with a dielectric material andplanarized, e.g., with a chemical mechanical polish (CMP) or etch toform shallow trench isolation region 34.

A bottom spacer 24 is formed at a base of the fins 18. The bottom spacer24 may include a nitride material although other suitable materials maybe employed. The bottom spacer 24 may be selectively deposited onhorizontal surfaces.

A gate dielectric 28 is formed on sidewalls of the fins 18. The gatedielectric 28 may include a grown oxide, a high-k material, a dielectricmaterial or combinations thereof. In various embodiments, the high-kmaterial of gate dielectric 28 may include HfO₂, HfSiO₄, HfSiON, La₂O₃,Ta₂O₅, ZrO₂, and/or SrTiO₃, or combinations thereof.

A gate conductor 26 is formed between the fins 18 on the gate dielectric28. The gate conductor 26 includes conductive materials, such as, e.g.,polycrystalline or amorphous silicon, germanium, silicon germanium, ametal (e.g., tungsten, titanium, tantalum, ruthenium, zirconium, cobalt,copper, aluminum, lead, platinum, tin, silver, gold), a conductingmetallic compound material (e.g., tantalum nitride, titanium nitride,tungsten silicide, tungsten nitride, ruthenium oxide, cobalt silicide,nickel silicide), carbon nanotube, conductive carbon, graphene, or anysuitable combination of these materials. The conductive material mayfurther comprise dopants that are incorporated during or afterdeposition. In useful embodiments, the gate conductor 26 may includeTiN, HfN, TaN, TiC, TaC, HfC, WC, TiAlN, W, etc. or combinationsthereof. The gate conductor 26 may include multiple layers including, awork function layer, a main conductor, diffusion barriers, etc.

The gate conductor 26 and the gate dielectric 28 are recessed below atop of the fins 18. A top spacer layer 30 is formed over the tops of thefins 18 and then the fins are exposed by a CMP or etch process. The topspacer layer 30 may include a nitride material although other suitablematerials may be employed.

A top S/D region 32 is grown on the exposed tops of the fins 18 and overthe top spacer layer 30. The top S/D region 32 may include Si and ispreferably doped in-situ. In this embodiment, the top S/D region 32includes a highly doped N-type region (N+). Each fin 18 now forms avertical field effect transistor (VFET) with S/D regions 14 or 20 and32. A gate structure including the gate dielectric 28 and the gateconductor 26 are disposed between the S/D regions 14 or 20 and 32. Inthe embodiment shown, three fins 18 are formed over the S/D layer 14,and three fins are formed over the S/D region 20 with the diode junction22 formed therebetween.

Referring to FIG. 5, an interlevel dielectric (ILD) 40 is formed overthe S/D region 32 and the bottom spacer layer 24. The ILD 40 may includeany suitable dielectric materials, e.g., a silicon oxide, nitride, etc.The ILD 40 is patterned to form openings for forming contacts 42, 44 and46. A contact 42 corresponds with the S/D region layer 14. Contacts 44land on the S/D region 32. One contact 44 is over the S/D region layer14 portion and the other contact 44 is over the P-type region 20. Acontact 46 connects to the gate conductor 26. The openings for thecontacts 42, 44 and 46 are formed by an etch process, e.g., a reactiveion etch (RIE) in accordance with a lithographic pattern. The RIEremoves the ILD 40 (and bottom spacer layer 24, where applicable) downto the epitaxy regions 14, 32 and gate conductor 26.

Contacts 42, 44 and 46 may include any suitable conductive material,such as polycrystalline or amorphous silicon, germanium, silicongermanium, a metal (e.g., tungsten, titanium, tantalum, ruthenium,zirconium, cobalt, copper, aluminum, lead, platinum, tin, silver, gold),a conducting metallic compound material (e.g., tantalum nitride,titanium nitride, tungsten silicide, tungsten nitride, ruthenium oxide,cobalt silicide, nickel silicide), carbon nanotube, conductive carbon,graphene, or any suitable combination of these materials. The conductivematerial may further comprise dopants that are incorporated during orafter deposition.

After the contact fill, a planarization process, such as a chemicalmechanical polish (CMP) may be performed. The planarization processcompletes the contacts 42, 44 and 46. Further processing may includeforming additional metal lines and connections.

Referring to FIG. 6, a test setup 60 is shown in accordance with oneillustrative embodiment. S/D region 14 includes a drain for verticaltransistors with fins 18 as channels for the DUT 50. Region 32 acts as asource for the DUT 50. A test structure 52 includes a portion of S/Dregion 32, the P-type region 20 and fins 18 as channels for verticaltransistors in the test setup region 52 (e.g., the three right hand sidevertical transistors in FIG. 6). During a test procedure, a sourcevoltage (Vs) of DUT 50 is biased to zero, e.g., Vs=0. A gate voltage(Vg) on gate conductor 26 and contact 46 is biased to turn on the DUT 50and force current through the source and drain of DUT 50, e.g., I_(ds).

Due to a reverse biased bottom junction 22, no current can flow in thefins 18 in the test setup side 52 (e.g., the three right side fins 18 inFIG. 6). In this way, no lateral current flows inside the S/D region 32over the fins 18 on the test setup side 52 (e.g., the three right sidefins 18 in FIG. 6). Therefore, the potential across the S/D region 32 isequal. For example, the S/D region 32 has equal potential across thetest setup side 52 and the DUT 50.

V_(meas) is a sense potential measurement of the S/D region 32. V_(meas)is measured at a probe contact 44 in the test structure 52. The contactresistance (R_(c)) can be calculated as R_(c)=V_(meas)/I_(ds) (whereI_(ds) is the drain-source current). If there were no P-type region 20at the bottom, current would flow laterally from right to left insidethe top S/D region 32, leading to a potential difference between the DUT50 portion of the S/D region 32 and the S/D region 32 in the test setupside 52. This would lead to erroneous measurement results. Instead, withthe uniform potential, an accurate measurement can be made for contactresistance using V_(meas) and I_(ds).

Referring to FIG. 7, a test setup 70 is shown in accordance with anotherillustrative embodiment. S/D region 14 includes a drain for verticaltransistors with a fin(s) 118 as a channel(s) for the DUT 50. Region 32acts as a source for the DUT 50. A test structure 52 includes a portionof S/D region 32, the P-type region 20 and a portion of fin 118 forvertical transistors in the test setup region 52.

Here, the fin 118 extends between the DUT 50 and the test structure 52.In one embodiment, the fin 118 is half in DUT 50 region over S/D layer14 and half in the test structure 52 and over P-type region 20.

As before, during a test procedure, a source voltage (Vs) of DUT 50 isbiased to zero, e.g., Vs=0. A gate voltage (Vg) on gate conductor 26 andcontact 46 is biased to turn on the DUT 50 and force current through thesource and drain of DUT 50, e.g., I_(ds).

Due to a reverse biased bottom junction 22, no current can flow in thefins 118 in the test setup side 52. In this way, no current flows insidethe S/D region 32 along the fin 118 on the test setup side 52.Therefore, the potential across the S/D region 32 is equal. For example,the S/D region 32 has equal potential across the test set up side 52 andthe DUT 50.

V_(meas) is a potential of the S/D region 32. The contact resistance(R_(c)) can be calculated as R_(c)=V_(meas)/I_(ds) (where I_(ds) is thedrain-source current). With the uniform potential an accuratemeasurement can be made for contact resistance using V_(meas) andI_(ds).

Referring to FIG. 8, methods for forming a test device areillustratively shown. In some alternative implementations, the functionsnoted in the blocks may occur out of the order noted in the figures. Forexample, two blocks shown in succession may, in fact, be executedsubstantially concurrently, or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved. It willalso be noted that each block of the block diagrams and/or flowchartillustration, and combinations of blocks in the block diagrams and/orflowchart illustration, can be implemented by special purposehardware-based systems that perform the specified functions or acts orcarry out combinations of special purpose hardware and computerinstructions.

In block 102, a diode junction layer is formed on a substrate. The diodejunction layer has a first dopant conductivity region and a seconddopant conductivity region formed within the diode junction layer onopposite sides of a diode junction. In block 104, a first portion ofvertical transistors is formed over the first dopant conductivity regionas a device under test (DUT), and a second portion of verticaltransistors is formed over the second dopant conductivity region. Thevertical transistors may be formed in a number of different ways havingdifferent structures for the DUT.

The vertical transistors may be formed vertically along semiconductorfins such that a device channel for the vertical transistors is disposedin a normal direction relative to the diode junction layer. The firstportion of vertical transistors may include whole fins, or the firstportion of vertical transistors may include a part of fins that extendsinto a device under test region and a part that extends into a testsetup region outside the device under test region.

In block 106, shallow trench isolation (STI) regions may be formed downto the substrate. In block 108, a common gate structure may be formedthat serves both the first and second portions of vertical transistors.

In block 110, a common source/drain region is formed over the first andsecond portions of vertical transistors such that current through thefirst portion of vertical transistors permits measurement (e.g., of aresistance) at a probe contact connected to the common source/drainregion. The common source/drain region is common to the DUT and the testsetup. During use, the diode junction actively prevents lateral currentflow in the common source/drain region when a source voltage of the DUTis maintained at substantially zero to measure the sense potential.

In block 112, contacts are formed through an ILD. The contacts include aprobe contact from which voltage measurements can be made (sensepotential). In block 114, processing continues to complete the device.The testing device may be formed on each chip or may be formed on one ormore chips of a wafer to provide data for all chips on the wafer.

Having described preferred embodiments for top contact resistancemeasurement in vertical FETs (which are intended to be illustrative andnot limiting), it is noted that modifications and variations can be madeby persons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method for forming a test device, comprising:forming a diode junction layer on a substrate, the diode junction layerhaving a first dopant conductivity region and a second dopantconductivity region formed within the diode junction layer on oppositesides of a diode junction; forming a first portion of verticaltransistors over the first dopant conductivity region as a device undertest and a second portion of vertical transistors over the second dopantconductivity region; and forming a common source/drain region over thefirst and second portions of vertical transistors such that currentthrough the first portion of vertical transistors permits measurement ofa resistance at a probe contact connected to the common source/drainregion.
 2. The method as recited in claim 1, wherein forming the firstportion of vertical transistors includes forming the verticaltransistors vertically along semiconductor fins such that a devicechannel for the vertical transistors is disposed in a normal directionrelative to the diode junction layer.
 3. The method as recited in claim2, wherein the first portion of vertical transistors includes wholefins.
 4. The method as recited in claim 2, wherein the first portion ofvertical transistors includes part of fins that extends into a deviceunder test region and part that extends into a test setup region outsidethe device under test region.
 5. The method as recited in claim 1,further comprising forming a common gate structure that serves both thefirst and second portions of vertical transistors.
 6. The method asrecited in claim 1, wherein the diode junction actively prevents lateralcurrent flow in the common source/drain region when a source voltage ofthe device under test is maintained at substantially zero to measure thesense potential.
 7. The method as recited in claim 1, further comprisingmeasuring current through the first portion of vertical transistors tomeasure a resistance at the probe contact connected to the commonsource/drain region.
 8. The method as recited in claim 1, furthercomprising forming contacts to the common source/drain region in thefirst portion and in the second portion including the probe contact. 9.The method as recited in claim 8, further comprising measuring contactresistance through the probe contact.
 10. A method for forming a testdevice, comprising: forming a diode junction layer on a substrate, thediode junction layer having a first dopant conductivity region and asecond dopant conductivity region formed within the diode junction layeron opposite sides of a diode junction; forming a first portion ofvertical transistors over the first dopant conductivity region as adevice under test and a second portion of vertical transistors over thesecond dopant conductivity region; and forming a common source/drainregion over the first and second portions of vertical transistors; andforming contacts to the common source/drain region in the first portionand in the second portion such that current through the first portion ofvertical transistors permits measurement of a resistance at a probecontact connected to the common source/drain region.
 11. The method asrecited in claim 10, wherein forming the first portion of verticaltransistors includes forming the vertical transistors vertically alongsemiconductor fins such that a device channel for the verticaltransistors is disposed in a normal direction relative to the diodejunction layer.
 12. The method as recited in claim 11, wherein the firstportion of vertical transistors includes whole fins.
 13. The method asrecited in claim 11, wherein the first portion of vertical transistorsincludes part of fins that extends into a device under test region andpart that extends into a test setup region outside the device under testregion.
 14. The method as recited in claim 10, further comprisingforming a common gate structure that serves both the first and secondportions of vertical transistors.
 15. The method as recited in claim 10,wherein the diode junction actively prevents lateral current flow in thecommon source/drain region when a source voltage of the device undertest is maintained at substantially zero to measure the sense potential.16. The method as recited in claim 10, further comprising measuringcurrent through the first portion of vertical transistors to measure aresistance at the probe contact connected to the common source/drainregion.
 17. The method as recited in claim 10, further comprisingmeasuring contact resistance through the contacts.